THE SECOND COMING OF THE KKKThe Ku Klux Klan of the 1920s and the American Political TraditionBy Linda GordonIllustrated. 269 pp. Liveright Publishing. $27.95.

The cover of Linda Gordon’s “The Second Coming of the KKK” shows a procession of men marching in full Klan regalia up Pennsylvania Avenue, the Capitol dome looming behind them. It would be a disturbing image in any era, but in 2017 — after the attack on an African-American church in Charleston, S.C., after the neo-Nazi demonstrations in Charlottesville, Va., after the alt-right poured into Washington for President Trump’s inauguration — it is terrifying.

The photograph was taken in 1925, during the decade when membership in the so-called Second Ku Klux Klan — the first was put down during Reconstruction — swept the country. In all, 30,000 men participated in that parade. What the photo leaves out are the throngs lining the avenue: The Klan didn’t just march in the nation’s capital; it received a warm welcome. Unlike the first and third Klans (the third appeared during the civil rights era), the 1920s Klan was well integrated into American life. “The K.K.K. may actually have enunciated values with which a majority of 1920s Americans agreed,” Gordon writes.

One of Gordon’s tasks is to show that the 1920s we think we know — a Gatsbyan bacchanal of speakeasies, flappers and mob hits — was just an urban, coastal bubble. For most Americans, it would appear, the decade was more like something out of “Babbitt” or “Elmer Gantry”: a country turned inward against the world, small-minded and cruel. A country in which the Klan and its values — so-called Americanism, xenophobia, white nationalism and patriarchy — were the norm. An America, Gordon all but says, not unlike today.

The second Klan was national in scope, with a surprisingly small footprint in the South — its highest per-capita state memberships were in Indiana and Oregon. In New Jersey, Klansmen burned a cross in the black section of Metuchen, today a liberal commuter suburb of New York. The Klan was so powerful in Southern California that it nicknamed Anaheim “Klanaheim.” Its main focus was, as always, on spreading hatred against blacks, Jews and Catholics, but its agenda always fit the local context: In the Southwest, it turned its ire on Hispanics and Latino immigrants; in the Pacific Northwest, it took aim at Japanese.

Like the alt-right today, the Klan was never a political party, but it wielded sizable influence in politics. Klan members or Klan-endorsed politicians held the governor’s office in Oregon, Texas and Colorado; it controlled mayor’s offices from Portland, Me., to Portland, Ore. And lest we criticize the current president for being uniquely unable to condemn the alt-right, bear in mind that no president in the post-World War I era from Woodrow Wilson to Herbert Hoover would condemn the Klan either, for fear of losing public support.

But the Klan’s real power lay not in politics but in its reach into the everyday. Gordon paints a picture like something out of a Vonnegut novel, an America seen in a fun-house mirror: The Klan sponsored baseball teams (one played the Hebrew All-Stars in a 1927 game in Washington, D.C.), county fairs (she includes a striking photo of Klansmen in full hooded regalia riding on a Ferris wheel in Colorado), college fraternities and beauty pageants, in which young women competed for the title of “Miss 100 Percent America.”

A historian at New York University, Gordon has written books on a broad range of topics, from 16th-century Ukraine to birth control, and she is one of only a few historians to twice win the Bancroft Prize, the profession’s highest honor. But in this book, she rejects the academic’s commitment to history for history’s sake in favor of a perspective on the past that explicitly comments on the present. “In my discussion of the Ku Klux Klan I am not neutral,” she writes, adding later in the same paragraph, “I am offering an interpretation, not a scholarly monograph.”

Gordon wants readers to consider the second Klan in light of recent American politics, but it’s important to parse what that era does and doesn’t say about our current situation. Today’s alt-right — and I use this term broadly, ranging from Richard Spencer and those even further to his right, to “mainstream” politicians, like the Senate candidate Roy Moore of Alabama and Representative Steve King of Iowa, who espouse a slightly laundered version of Spencer’s nationalist, traditionalist xenophobia — is nowhere near as broad or pervasive as the Klan was then.

Nor is the alt-right as well organized. Readers’ jaws will drop at how expansive and structured the Klan was, from its code words to its machine-like control over City Councils and state legislatures. In many small towns, belonging to the Klan was a means of career advancement for political strivers — among them Hugo Black and, apparently, Harry Truman. It’s hard to imagine anyone, anywhere, saying the same of the alt-right today.

But underneath those differences are similarities that point to a recurrent tendency in American history. It’s hard to finish a single page in Gordon’s book without a slight tingle of fearful familiarity, of reverberations in rhetoric and public opinion — a recognition that, maybe, it has always been thus. “Precisely because the second Klan was so mainstream,” she writes, “examining it also reveals continuing currents in American history, currents at times rising to the surface, at other times remaining subterranean.”

Like today’s alt-right, the second Klan envisioned an American past cut from mythical cloth: an America without immigrants, an America ruled by Anglo-Saxon whites, an America that prayed in unison to an evangelical-Christian God. The Klan rejected scientific claims that challenged its worldview. It railed against the cosmopolitan, liberal elite, but it tried to make common cause with moneyed interests. It played on white people’s sense of “fear, humiliation and victimization.” And it spread misinformation about its enemies, planting false news stories about conspiratorial Jews and greedy Catholic priests. These echoes are not coincidental.

The second Klan fell as fast as it rose; with several million members at its height in the mid-1920s, it had collapsed later in the decade to 350,000, brought low by internal corruption scandals. Something similar could, perhaps, happen today. Should a scandal turn Steve Bannon and Breitbart toxic, is there any doubt that their movement would suffer a body blow?

There are two ways to think about this. One could say, great — we’ve met the enemy before, and defeated him. We’ll do it again. Or we could realize that we’ve met the enemy, and he is us. That the plague of xenophobia, racism and nationalism is always present, “that it can lie dormant for years and years in furniture and linen-chests,” as Camus wrote, ready to re-emerge, given the right conditions.

They say the job of an anthropologist is to make the familiar strange and the strange familiar, and something similar goes for the historian. I can think of few books that accomplish this task as well as Gordon’s: In her telling, the second Klan is at once utterly bizarre and undeniably American. The 2010s may not be the 1920s, but for anyone concerned with our present condition, “The Second Coming of the KKK” should be required reading.

Clay Risen is the deputy Op-Ed editor for The Times. He is writing a book about Theodore Roosevelt and the Rough Riders.